The invention relates to methods and devices for ascertaining the rotary speed of a motor, particularly a permanent-magnet-stator D.C. motor.
It is very well known to ascertain the rotary speed of a motor using means which cannot be kept out of physical contact with the motor. The most familiar of these is of course a tachometric generator whose rotating structure is mounted on the motor shaft for rotation therewith and which is operative for generating an output signal indicative of the rotary speed of the motor. However, using a tachometric generator to ascertain rotary speed subjects the motor to an additional mechanical load, which may be disadvantageous in itself and/or may detract from the accuracy of the rotary-speed information required in certain practical applications.
It is also known to ascertain the rotary speed of a motor using means which need not physically contact the motor. The most familiar of these is a photoelectric speed detector. A bright marking, or the like, is provided on the output shaft of the motor and repeatedly moves past a stationary photoelectric sensing device. The output signal of the photoelectric sensing devices is comprised of repeated dark-light and light-dark transitions, and is typically integrated with respect to time before it can be converted into an actual rotary-speed indication.